NetBSD is a very portable BSD-licensed operating system.
This is CoreCLR, the open source .NET framework from Microsoft.

Why is this important?
Programs for NetBSD can be run in a stripped down NetBSD kernel and libc (a unikernel) called rumpkernel, which can be run on any platform, including on bare metal.
It is also extremely minimal - an operating system image for nethack was a mere 4MB.

Rump can also run on any architecture that NetBSD runs on (including MIPS, VAX, m68k, as well as standard platforms like amd64 and ARM) and also on those without fully NetBSD support, but just with a functional C compiler (like RISCV).
Many people use rump as an extremely lightweight container - it can run on Linux and others.

Rump was recently used to port NetBSD audio drivers to GNU/Hurd.

Caveats: doesn’t run in rump yet, but this is a big step towards it. .NET is also not self-hosting yet, and requires assemblies (CoreFX) to be cross-compiled from Linux.

The work was done almost entirely by Kamil Rytarowski, a NetBSD developer, in his free time. Good job Kamil!